An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
A white-emitting electroluminescent (EL) layer can be used to form a multicolor device. Each pixel is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340.
White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color. The following patents and publications disclose the preparation of organic OLED devices capable of producing white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between a pair of electrodes.
White light producing OLED devices have been reported by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host-emitting material. Sato et al. in JP 07-142169 discloses an OLED device, capable of emitting white light, made by forming a blue light-emitting layer next to the hole-transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent layer.
Kido et al., in Science, 267, 1332 (1995) and in Applied Physics Letters, 64, 815 (1994), report a white light-producing OLED device. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. More recently, Deshpande et al., in Applied Physics Letters, 75, 888 (1999), published a white OLED device using red, blue, and green luminescent layers separated by a hole-blocking layer.
Anthracene based hosts are often used. A useful class of 9,10-di-(2-naphthyl)anthracene hosts has been disclosed in U.S. Pat. No. 5,935,721. Bis-anthracene compounds used in the luminescent layer with an improved device half-life have been disclosed in U.S. Pat. No. 6,534,199 and U.S. Patent Application Publication 2002/0136922 A1. Electroluminescent devices with improved luminance using anthracene compounds have been disclosed in U.S. Pat. No. 6,582,837. Anthracenes have also been used in the hole-transporting layer (HTL) as disclosed in U.S. Pat. No. 6,465,115. In addition, there are other disclosures of using anthracene materials in OLED devices, U.S. Pat. No. 5,972,247, JP 2001-097897, JP 2000-273056, U.S. Patent Application Publication 2002/0048687 A1, WO 03/060956 A2, WO 02/088274 A1, EP 0429821, WO 03/007658 A2, JP 2000-053677, and JP 2001-335516.
Despite these advances, there is a continuing need for hosts and dopants that provide better operational stability and are conveniently manufactured. Improved operational stability of OLED devices will permit their use in more products.